1. Field of the Invention
The present invention relates to an optical encoder with a scale optically scanned by a scanning device.
2. Description of the Prior Act
Such encoders are particularly used with machine-tools for measuring the relative movement of a tool with respect to a machined workpiece, and with coordinate-measuring machines. Upon this use of the encoders, the encoders are subjected to action of electrical fields, in particular parasitic electrical alternating fields which influence the operation of the scanning device and cause generation of electrical disturbance signals and, as a result, erroneous position determination.
There are known different measures that were undertaken to shield the particularly interference-prone sensor elements of the scanning device from the electrical fields.
According to European Patent Publication EP 0328661 B1 from which the present invention proceeds, the sensor elements are located in a grounded housing. The housing is covered with a transparent scanning reticle in front of the light-sensitive surfaces of the sensor elements. A scanning pattern, which is formed of an opaque, electrically conductive material and which also grounded, is provided of the scanning reticle. The scanning pattern limits transparent windows which are arranged in predetermined phase positions for generating phase-shifted, relative to each other, electrical scanning signals. These measures result in an adequate screening only when the scanning pattern has sufficiently small windows.
In order to achieve a complete screening, German Patent DE 44 02 554 suggests to screen the light-sensitive region of the sensor elements with a transparent screen member which is formed of an electrically conductive material and is grounded. It is suggested to use, as a screen member, a transparent foil provided with a transparent conductive layer. The layer can be formed of thin metal layers or conductive metal oxide (ITO) layers. It is also suggested to form a scanning pattern on such a screen member or to secure thereto a scanning reticle.
The drawback of this measure consists in that such screen member has only a relatively small light transmission (about 85%).
Accordingly, an object of the present invention is to provide an optical encoder having a high interference resistance against electrical fields, in particular, parasitic electrical fields.
Another object of the present invention is to provide an optical encoder with screening means that would disturb the optical beam path in the encoder as little as possible.
These and other objects of the present invention, which will become apparent hereinafter, are achieved by providing an optical encoder of the type described above in which a scanning device, which is displaceable relatively to the scale in a measuring direction, has a scanning reticle with at least one transparent window limited by an opaque screen and a partial grit formed of an electrically conductive material and provided at least in region of the window.
The foregoing novel features of the present invention permit to achieve good screening of the light-sensitive sensor, with a minimal distortion of the optical beam path.
The novel features of the present invention, which are considered as characteristic for the invention, are set forth in the appended claims. The invention itself, however, both as to its construction and its mode of operation, together with additional advantages and objects thereof, will be best understood from the following detailed description of preferred embodiments, when read with reference to the accompanying drawings.